1. Field of the Invention
This invention relates to ion mobility spectrometers and more particularly to a miniature ion mobility spectrometer.
2. Description of the Prior Art
The technique of ion mobility spectrometry (IMS) was disclosed in the early 1970's in order to analyze and detect organic vapors. A typical ion mobility spectrometer (IMS) detector cell consists of a reaction region for generating ions and a drift region for measuring the mobility of the ions. The reaction region and the drift region are separated by a shutter grid which functions to gate the ions from the reaction region into the drift region. An inlet duct provides a gas to the reaction region. In the reaction region an ionization source, for example, tritium, 63.sub.Ni, 241.sub.Am, etc. ionizes some of the gas. An electric field in the reaction region moves the ions towards the shutter grid. The ions are gated into the drift region where their mobility is measured by their arrival times at a collector at the far end of the drift region. The drift region is biased with an electric field to move the ions towards the collector. The drift region may have a drift gas flowing therein from the collector towards the shutter grid.
One example of a means for providing an electric bias field in the drift region as well as the reaction region is provided in U.S. Pat. No. 4,390,784 which issued on June 28, 1983 to D. R. Browing et al. and assigned to The Bendix Corp., now merged with the assignee herein. In U.S. Pat. No. 4,390,784 a ceramic tube is disclosed with a coating on its inside surface of a thick film resistor. A voltage potential is placed across the film in the longitudinal direction of the drift region to provide a longitudinal electric field from the shutter grid to the collector.
In U.S. Pat. No. 4,445,038 which issued on Apr. 24, 1984 to G. E. Spangler and J. F. Wroten, Jr. and assigned to The Bendix Corporation, now merged with the assignee herein, an ion mobility detector is described having dual drift regions respectively on either side of a centrally located reaction region and respectively separated from the reaction region by dual shuttered grids.
An improvement to the inlet of gas to the reaction region is described in U.S. Pat. No. 4,311,669 which issued on Jan. 19, 1982 to G. E. Spangler entitled "Membrane Interface for Ion Mobility Detector Cells" which is assigned to The Bendix Corporation, now merged with the assignee herein. In U.S. Pat. No. 4,311,669, a membrane interface is provided over the sample inlet port of an ion mobility detector. The sample penetrates the membrane and is carried into the ion mobility detector by means of a carrier gas which scrubs the interior surface of the membrane.
Another improvement to the inlet of gas into the reaction region is described in U.S. Pat. No. 4,378,499 which issued to G. E. Spangler et al. which is assigned to The Bendix Corporation now merged with the assignee herein entitled "Chemical Conversion for Ion Mobility Detectors Using Surface Interactions". In U.S. Pat. No. 4,378,499 the inlet gas including a sample is passed through a catalytic reactive filter prior to injection into the reaction region of an ion mobility spectrometer to enhance selectivity and sensitivity by converting through surface interactions sample gas or vapor to a form more readily ionized or by converting through surface interactions interferent gas or vapor to a form less readily ionized. Alternatively, the inlet gas may be exposed to a reactive coating placed on the surface of a permeable membrane prior to diffusion therethrough into the reaction region or exposed to a reactive coating placed on the interior walls of the reaction region. The reaction region is formed by a plurality of conductive rings and a drift region formed by a plurality of conductive rings with drift gas entering near the detector and being exhausted near the shutter grid.
It is therefore desirable to provide an ion mobility spectrometer which is easy to assemble and miniature in size.
It is further desirable to provide an ion mobility spectrometer which has a metal housing surrounding a conductive inlaid tube (CIT) which provides an electrical and thermal shield to the CIT.
It is further desirable to provide an ion mobility spectrometer with a CIT with heater elements affixed to the outside surface for heating the CIT and for heating the gas entering the drift region.
In is further desirable to provide an ion mobility spectrometer wherein the drift and carrier gases are exhausted through holes in the shutter assembly and the conductive ring adjacent thereto to allow counter-flow of gases in the drift region and to prevent signal degradation as a result of gas mixing and dilution of the carrier gas and drift gas in the reaction region.
It is further desirable in an ion mobility spectrometer to provide an aperture and collector configuration designed as a Faraday shield enclosing the collector so that relative motion between aperture grid and collector is compensated by motion of opposite polarity and that effects of microphonic noise are reduced.